The present invention relates generally to a method and apparatus for connecting an initial length of add-on piling to a length of piling suspended from an offshore tower and for thereafter handling the entire piling without imparting undue impact loadings on the derrick employed to manipulate the piling. More particularly, the invention relates to a method and apparatus by means of which the alignment and connection of the add-on piling to the suspended length of piling is automated to a large extent and through which the entire piling can be lowered to the floor of a body of water without imposing the impact loads mentioned.
In recent years a great deal of commercial activity has begun focusing on the oceans. In particular, as supplies of petroleum indigenous to the land masses of major industrial countries have diminished exploration of the geological strata underlying the oceans has intensified. Such exploration is continuing and is being pressed into even deeper areas of the oceans.
Once located, oil fields submerged beneath bodies of water may be exploited. This exploitation is commonly initiated through the use of enormous offshore towers positioned in the oil field. The jackets, i.e. the basic framing portions of these towers, are most commonly constructed in a graving dock in a nearby shoreline and are thereafter floated to the desired location. Once the desired location is reached, the jacket is turned upright and placed on the floor of the body of water. A number of pilings are then forced into the floor of the body of water to anchor the jacket in place and the upper portions of the structure are put in place. Once the tower has been completed, one or more wells can be directionally drilled therefrom. A representative offshore tower which may be employed is disclosed in Crout et al. U.S. Pat. No. 3,823,564 which is assigned to the assignee of the present invention. Because of the useful background present in this reference, the text of this patent is hereby incorporated herein by reference as though set forth at length.
As can be well appreciated from an examination of the Crout et al. patent mentioned in the preceding paragraph, offshore towers used in the exploration for oil are normally characterized by a plurality of long legs interconnected by lattices of bracing members. Pilings employed to anchor the tower to the floor of the body of water can interact with the legs in at least two different ways. The pilings may, for instance, pass downwardly through the legs into the floor of the body of water or alternatively pilings may pass downwardly through piling guides disposed on the exterior of the legs. The piling guides used in connection with the latter approach may take the form of conduits and/or guiding collars. In either of the above cases, the length of piling which is to be initially driven into the floor of the body of water may be secured temporarily in place during the constructions of the jacket. The initial lengths of piling to be driven can thus be carried with the jacket as it is floated to the desired offshore location as essentially an integral part of the jacket. The amounts of piling which must be hauled to the site by a service barge are significantly reduced and thus the logistics of the placement of the tower can be simplified. The lengths of piling normally extend from either the lower end of the jacket or from a short distance thereabove to near the top of the jacket.
The jacket is first floated to the desired site and placed on the floor of the body of water. While the initial lengths of piling to be driven are still temporarily connected to and thus suspended from the jacket, add-on pilings may be lifted into place by means of a derrick floating nearby and may be connected to the suspended length of piling. Once the add-on pilings are connected to the pilings suspended from the tower, the entire assembly can be released from connection with the tower and moved downwardly into contact with, or the short distance through the mud into the submerged surface water. At this point the pilings are normally driven into the floor of the body of water and may ultimately be grouted to solidly anchor the jacket.
Quite significantly, the add-on pilings must be permanently connected to the pilings initially suspended from the tower and must also be properly aligned therewith to ensure smooth and complete penetration of the strata underlying the tower. Structural inadequacies in the connection between the add-on and the suspended lengths of piling, as well as variations in the axial alignment thereof, may precipitate a failure of the pilings and/or may diminish the extent to which the piling can be driven into the floor of the body of water.
It should thus be apparent at this point that once the jacket is placed on the floor of the body of water it should be possible to effectively connect the add-ons to the temporarily suspended lengths of piling, to then release the pilings from the temporary connection with the tower, and to thereafter drive and possibly grout the pilings securely in place. These exacting requirements must be met under grave handicaps. Workmen erecting the tower must contend with a very hostile environment, particularly if the tower is to be placed in such an area as the North Sea. Furthermore, the amounts of equipment available in placing the tower are relatively limited since normally most of the equipment must operate from vessels floating nearby. This equipment, of course, is subject to the hazards of the open sea and due to the turbulent character of the waters may be vulnerable to overstressing. Finally, it must be realized that once the add-on pilings are connected to the suspended pilings, the piling, often extremely heavy, must be released from the structure and safely moved into position preparatory to the forcing of the piling into the underlying strata.
As was mentioned earlier, the pilings suspended temporarily from the offshore tower may extend to the intended base of the tower or to within a relatively short distance thereof. The piling thus does not quite reach solid underlying strata of the floor of the body of water capable of supporting the piling. Thus, once the add-on is connected and the whole piling disconnected from the tower, if the derrick used to manipulate the add-on remains connected to the piling, the derrick may be subjected to severe impact loading as the full piling load is assumed by the derrick. It would therefore be highly desirable if a method and apparatus could be provided in which the floating derrick is largely isolated from impact loads which may occur as an incident to the anchoring of the tower to the floor of the body of water.
It may be necessary in many methods and apparatus of the prior art for the floating derrick to steady and/or align the add-on pilings as each is connected to the piling temporarily suspended from the jacket. While the add-on is being connected to the suspended piling, it may be necessary to maintain the cable extending between the boom of the jacket and the add-on piling in tension. During this portion of the operation, winds and the action of waves may cause movement of the derrick relative to the tower. Sudden movements of this type, particularly if the add-on is in any way restrained by the jacket, can impart severe impact loadngs to the derrick. This may be a particular problem if the waves occurring in the vicinity of the jacket cause the derrick to heave significantly. It will thus be appreciated that it would be quite desirable if a method and apparatus could be provided which would permit the disconnection of the derrick from the add-on during connection of the add-on to the suspended piling and prior to the time at which the suspended piling is disconnected from the jacket. This would avoid impact loadings imposed as an incident to the action of waves in the vicintiy of the jacket.
Certain methods and apparatus of the prior art may require the piling temporarily suspended from the jacket to be disconnected therefrom without any concurrent restraint. This disconnection could precipitate a sudden dropping of the piling to the floor of the body of water. Inasmuch as the piling can be quite heavy, this clearly poses a threat of damage to the jacket and injury to workmen. This problem might be circumvented to some extent by restraining the piling by means of the floating derrick. This, however, would cause either or both of the problems discussed in the preceding paragraphs. It would therefore be very advantageous if a method and apparatus could be provided wherein the piling is restrained as it is being disconnected from the jacket and wherein the piling can be safety lowered without imposing severe impact loadings on the floating derrick.
Proper connection of the pilings together may require a particular rotational relation between the add-on piling and the suspended length of piling. It would therefore be highly desirable if a method and apparatus could be provided which would afford an accurate, safe, and rapid as well as essentially automated alignment of the add-on piling relative to the suspended length of piling without the need of cumbersome external fixtures or other structure integral with the jacket.
Commonly, in methods and apparatus of the prior art, the add-on pilings are lifted to atop the suspended piling preparatory to undergoing connection therewith. Once the add-ons are so placed, workmen must contend with a long, slender length of very heavy material poised atop the tower, fully exposed to a hostile environment and perhaps subject to movements of the floating derrick. It would therefore be very advantageous if a method and apparatus could be provided which would rigidly, but adjustably, connect and stabilize an add-on piling atop a suspended length of piling preparatory to and during actual connection therebetween.